1. Field of the Invention
The present invention relates to a liquid crystal display device. In particular, the present invention relates to a liquid crystal display device with an excellent viewing angle, a method for producing such a device, and an apparatus for producing such a device.
2. Description of the Related Art
Conventionally, a liquid crystal display device (hereinafter referred to as "LCD") includes liquid crystal molecules in a liquid crystal layer interposed between a pair of substrates. When the alignment of the liquid crystal molecules is changed, the optical refractive index of a liquid crystal layer is also changed. By utilizing the change in the refractive index, the LCD performs the display. Accordingly, it is important to control the alignment of the liquid crystal molecules in the liquid crystal layer. The liquid crystal molecules are specifically arranged by the interactions between the surfaces of the substrates sandwiching the liquid crystal layer and the liquid crystal molecules.
In the method which is currently the most widely used for aligning liquid crystal molecules in a certain direction: an alignment film is formed on each of the surfaces of the substrates which face the liquid crystal layer; then the surfaces of the alignment films are rubbed.
For example, a polyimide film formed on the substrate is subjected to a rubbing treatment in order to align the liquid crystal molecules along the rubbing direction. Since the conventional rubbing treatment is unidirectionally performed, the liquid crystal molecules in the vicinity of the surface are unidirectionally aligned. Therefore, a viewing direction of the liquid crystal display device is defined by the orientation of the liquid crystal molecules in the liquid crystal layer, that is, the rubbing direction.
The liquid crystal display device produced by such a rubbing method has problems in viewing angle characteristics because the liquid crystal molecules in the center portion in a thickness direction of the liquid crystal layer are unidirectionally aligned. In such a case, the following disadvantageous problems occur.
In a liquid crystal display device using a thin film transistor (TFT-LCD), a structure called Twisted Nematic (hereinafter, referred to as "TN") mode is adopted. FIGS. 10 and 11 are a perspective view showing a schematic structure of a conventional liquid crystal display device and a cross-sectional view thereof. In FIGS. 10 and 11, a positive viewing direction 1 is a major axis direction of liquid crystal molecules 2 in the vicinity of the central portion of the liquid crystal layer, that is, a direction in which the major axis direction of the liquid crystal molecules is orientated when the liquid crystal molecules rise. An angle .delta. indicates a pretilt angle of the liquid crystal molecules 2. Moreover, a rubbing direction 5 of a substrate 3 and a rubbing direction 6 of a substrate 4 are perpendicular to each other so that the liquid crystal molecules 2 are twisted at 90.degree. between the pair of the substrates 3 and 4. A transparent electrode 3b and an alignment film 3c are formed on the glass substrate 3a, thereby constituting the substrate 3. A transparent electrode 4b and an alignment film 4c are formed on the glass substrate 4a, thereby constituting the substrate 4.
In a normally white (hereinafter referred to as "NW") mode in which the liquid crystal display device transmits light therethrough (i.e., displays white) when the voltage is not applied. Voltage-transmittance (V-T) characteristics are shown in FIG. 12. As indicated with a solid line L1 in FIG. 12, when the liquid crystal display device is viewed from the normal to the substrate, the light transmittance decreases as the applied voltage increases.
When the viewing angle is inclined in the positive viewing direction from the normal to the substrate of liquid crystal display device, the V-T characteristic is as indicated with a solid line L2 in FIG. 12. There is a voltage region where the transmittance increases as the applied voltage increases. In this voltage region, an inversion of gray-scale, that is, an inversion of black and white occurs. This phenomenon is caused by the change in the apparent refractive indices due to the viewing angle. In the NW mode, the darkest display is obtained when the liquid crystal display is viewed along a direction perpendicular to the substrate while the major axes of the liquid crystal molecules are aligned parallel to the viewing angle and in a direction perpendicular to the substrate. The liquid crystal molecules are inclined (tilted) in accordance with each voltage corresponding to each gray-scale. When the viewing angle is inclined from the direction perpendicular to the substrate, the viewing direction deviates from the major axis direction of the liquid crystal molecules in the region displaying black (i.e., aligned in the direction perpendicular to the substrate), to become parallel to the major axis direction of the inclined liquid crystal molecules of the region displaying half-tone. As a result, the half-tone displaying region looks darker than the region displaying black.
As shown in FIG. 13A, when the voltage applied across the transparent substrates 3 and 4 is zero or a relatively low voltage, the center molecule 2 of the liquid crystal layer is observed as an ellipse by a viewer 7 positioned in the positive viewing direction 1. As the applied voltage is gradually increased, the center molecule 2 rises to be parallel to the electric field direction. Accordingly, as shown in FIG. 13B, the center molecule 2 at a certain voltage is observed as a circle by the viewer 7. As the applied voltage is further increased, the liquid crystal molecule 2 becomes substantially parallel to the electric field direction. As a result, the liquid crystal molecule 2 is observed again as ellipse by the viewer 7, as shown in FIG. 13C.
Similarly, in the viewing directions other than the positive viewing direction 1, V-T characteristics vary depending on the viewing directions. In the directions other than the positive direction, the inversion phenomenon does not occur since the viewing directions do not agree to the major axis direction of the liquid crystal molecules. However, there is a problem that the contrast ratio is lowered as the viewing angle is inclined, as indicated with a solid line L3 of FIG. 12. The above-mentioned problems such as gray-scale inversion and unevenness of the viewing angle characteristics always occur not only in the TN mode LCD but also in a display device which has one orientation state over the entire display panel and conducts display by utilizing birefringence of the liquid crystal molecules.
A technique for obtaining a TN mode LCD with improved viewing angle characteristics is disclosed in Japanese Laid-Open Patent Publication No. 5-173135. According to the technique, the surface of the alignment film is unidirectionally rubbed, and then a resist is applied on a part of the alignment film. Then, the rubbing treatment is performed in the direction reverse to the previous rubbing direction. As a result, the alignment film is provided with different alignment conditions caused by the different rubbing directions between the alignment film surface covered with the resist and the alignment film surface not covered with the resist, so as to provide the liquid crystal cell with opposite viewing directions.
However, if the conventional method for rubbing the surface of the alignment film a plurality of times is used, each of the upper and lower substrates should be subjected to a rubbing treatment twice, a resist application treatment and a resist removing treatment, in order to provide the liquid crystal molecules in one pixel with two orientation directions. In the case where the alignment film is divided into the larger number of regions in order to improve the viewing characteristics, the number of rubbing treatments and the number of other treatments are further increased.
In this way, if the rubbing method is used as a means for adding a plurality of alignment directions to one pixel, there are problems such as a complicated fabrication process, and contamination and deficiency of the alignment film caused in the photolithography process. Therefore, this method is not suitable for mass production.